Something very notable happened over the weekend in the debate on immigration. A House Republican joined Democrats in support of an immigration plan that includes a path to citizenship for illegal immigrants.

To understand why is to understand the political outlook of Rep. Jeff Denham (R-Calif.), the Republican who embraced the plan. Denham's decision -- regardless of his motivation -- is good politics. There are few other House Republicans who could say the same thing, which suggests he's probably not the first of a wave of GOP crossovers, but rather, an outlier.

Rep. Jeff Denham (R-Calif.) (J. Scott Applewhite / AP)

Call it the triple threat. Denham represents a district where (1) President Obama won, (2) there's a large Hispanic population and (3) a serious Democrat is challenging him. In short, Denham must by political necessity walk a much more moderate line on immigration than almost all of his Republican colleagues.

That's exactly what he's doing by joining 185 Democrats in support of a measure that adopts most of a sweeping bill that cleared the Senate earlier this year, which Denham took some issue with in August. The measure also includes Denham's proposal to allow undocumented immigrants to attain legal status through service in the U.S. armed forces.

"I’m the first Republican," he told the Washington Post's Peter Wallsten. "I expect more to come on board."

More Republicans may indeed join Denham, but it would be surprising if many of them do, simply because Denham is in a pretty unique situation.

The Cook Report's Amy Walter took a look in June at all of the House Republicans who represent districts where the Hispanic share of the population is 25 percent or greater. (Hispanics tend to favor a path to citizenship for undocumented immigrants and are impacted heavily by immigration laws, which is why these districts are worthy of a closer look.)

What she found illustrates why immigration reform is so tough for the House to pass.

For starters, there are only two dozen such districts. And of the two dozen, 15 were carried by Mitt Romney by double digits -- in other words, districts with strong conservative leans, and opposition to a path to citizenship is strong among conservatives.

Denham's district is 40 percent Hispanic and Obama won there by four points. Of the districts Walter looked at, only three went to Obama by a greater margin. And in just six (including those three), Hispanics make up a larger share of the overall population than they do in Denham's district.

If more Republicans follow Denham's lead, they could well come from this pool, which includes California Reps. Gary Miller and David Valadao, as well as Florida Rep. Mario Diaz-Balart. Miller and Valadao are vulnerable in 2014 and Diaz-Balart is part of a bipartisan group working to craft a compromise reform bill. The other three are Reps. Steve Pearce (N.M.), Ileana Ros-Lehtinen (Fla.), and Blake Farenthold (Tex.), who would surprise many if he joined Democrats.

There are other possibilities, too. (Wallsten reports that advocates are circulating a list of 28 target House Republicans.) Who else might be willing to cross over? Look to the districts that groups favoring reform have targeted with ads in recent months. Reps. Mike Coffman (R-Colo.), Joe Heck (R-Nev.), Daniel Webster (R-Fla.) are three names worth watching, among others. They have been pressed from the left on immigration and face potentially competitive reelection campaigns.

Denham's move is indisputably progress for proponents of immigration reform, but the list of potential Denhams-in-waiting is not very long. And to seriously press House GOP leadership to act, Democrats would have to round up enough Republicans to get the total number of pledged votes for their plan to a majority of the House. Even if every House Democrat were to sign on -- which has not happened yet -- as many as 17 Republicans would need to join them, depending on when the vote is held. That's a ways to go, and even then, GOP leaders who have already said they don't favor the Senate's sweeping bill may not agree to hold a vote.

While polls have shown pretty broad support for a path to citizenship, the most important political factor for Republican incumbents is how it's playing back home. And with most Republicans sitting in conservative districts -- in large part due to redistricting -- what plays well broadly is not the same as what plays well back home.